It is currently estimated that there are between 3 and 12 billion dollars a year of natural gas was lost to flare off due to an inability to capture, refine, and/or transport it effectively. While methods are known for capture, refinement and/or transport of natural gases, they tend to be cumbersome and not readily amenable for use in remote or offshore natural gas deposit locations.
Natural gases may be converted to liquid fuels by a variety of known methods. For example, such methods include Fischer-Tropsch and Mobil Processes, as well as plasma-assisted gas-to-liquid (GTL) techniques. The Fischer-Tropsch and Mobil Processes involve multi-stage synthetic steps where a light hydrocarbon (i.e. hydrocarbon gas) is initially transformed to syngas, under high pressure and high temperatures of up to 1300 K. Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2). It is typically formed by oxygen-deprived combustion of the hydrocarbon gas. The following reactions are exemplary of these well-known processes:CO+H2→liquid hydrocarbons (Fischer-Tropsch process)CO+H2→CH3OH and/or other liquid hydrocarbons (Mobil Process)
Because of the extreme thermal operating conditions, syngas reformers are massive to build and are expensive to operate. GTL plants, in order to be commercially viable, need to be very large and complex. High operating energy consumption is required for gas compression and heating, and accounts for approximately 60-80% of all costs for fuel production by such methods. Furthermore, generally expensive reformation catalysts are used in all stages of the conventional processes, and require catalyst recovery from the systems.
Another approach to the conversion of light hydrocarbons to liquid fuels is via a non-thermal plasma-assisted method. U.S. Pat. No. 7,033,551 (the '551 patent) discloses a reactor system having an electrochemical cell and a dielectric barrier discharge, where the formation of liquid products occurs primarily through the oligomerization of gaseous hydrocarbon radicals in a non-thermal plasma of a barrier gas discharge. Non-thermal plasmas provide an initial radical concentration via dissociation of light alkane molecules by energetic electrons at low gas temperature (about 100° C. to about 600° C.) under atmospheric gas pressure. Electrochemical cells in conjunction with the barrier discharge allows for oxidation of excess hydrogen in the plasma, partial oxidation, and oxidative condensation of the primary gas. The final composition includes a mixture of liquid hydrocarbons, of which a minority are alcohols.
The method described in U.S. Pat. No. 7,033,551 is based on the implementation of dissociation processes that occur under the action of “hot” electrons on hydrocarbon molecules inside the barrier discharge reactor according to reaction (1):e−+RH→R.+H.+e−  (1)
In reaction (1), RH is a general formula for a hydrocarbon and e− is an electron. The radicals R. and H. are formed at high activation energies (>400 kJ/mol) in such processes. Similar processes with a similarly high activation energy may also be facilitated through a light-assisted process, where an ultra-violet (UV) radiation source (hv) provides the requisite energy, as described by the '551 patent:hν+RH→R.+H.  (2)
The large activation energy requirement for reactions (1) and (2) is due to the energy state of the unactivated hydrocarbon molecule lying at a level that is much lower than the energy state of its dissociated components. Each bond breaking event (i.e. dissociation) through electron impact takes place only via electronic state excitation, and in doing so consumes a significant amount of energy. Taking into account the energy released on reformation of higher hydrocarbons (reaction (3)) after the dissociation reactions above:2R.→R2  (3)the energy consumption for the process typically is higher than 100 kW*h per 1 kg of the end product.
U.S. Pat. No. 6,375,832 (the '832 patent) discloses the synthesis of liquid products under the action of a barrier discharge, while the use of a catalyst is optional. In the synthetic process described by the '832 patent, oligomers of hydrocarbon radicals are produced as a result of dissociation of the feed gas, and reformation of hydrocarbons from free radical fragments through direct coupling and oxidative condensation:CH4→C2H6→C4H10  (4)
If CO2 is introduced into the feeding gas mix as an oxidant, then carbon dioxide conversion also occurs and contributes to the formation of the liquid hydrocarbons. Alcohols may also be produced as a result of CO2 decomposition. Such processes are summarized by reactions 5-7:CO2+e−→CO+O.+e−  (5)RH+O.→R.+OH  (6)R.+OH.→ROH  (7)
Limiting factors of the above plasma-assisted methods are: the non-chain character of conversion processes in the barrier discharge reactor and the high activation energy (>400 kJ/mol) of the primary radical formation process. Consequently, the specific energy consumption for the production of liquid products commonly exceeds 100 kW*h per 1 kg of product. Another significant limitation of the barrier discharge plasma-assisted methods is the low current (10−5-10−3 A/cm2) and power density of the barrier discharge plasma (1-10 W/cm3), which reduces the capability of the reactor systems. Furthermore, the above plasma-assisted methods control only the feed gas temperature.
A plasma chemical reactor may be used for converting gaseous hydrocarbons to liquid fuels as described in co-pending application 2011/0190565. With regard to processing gaseous hydrocarbons such as natural gas, there are a large amount of non-hydrocarbon components such as nitrogen. In some cases, nitrogen may take up to 25% of total volume of the gaseous hydrocarbon, which reduces the efficiency of the plasma chemical reactor. Thus, there is a need to build a system that is more efficient for converting gaseous hydrocarbon to liquid fuel.